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<div><h2 class="title">
<a name="boost_localfunction"></a>Chapter&#160;1.&#160;Boost.LocalFunction 1.0.0</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Lorenzo</span> <span class="surname">Caminiti <code class="email">&lt;<a class="email" href="mailto:lorcaminiti@gmail.com">lorcaminiti@gmail.com</a>&gt;</code></span>
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<div><p class="copyright">Copyright &#169; 2009-2012 Lorenzo
      Caminiti</p></div>
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<a name="boost_localfunction.legal"></a><p>
        Distributed under the Boost Software License, Version 1.0 (see accompanying
        file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="index.html#boost_localfunction.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/getting_started.html">Getting Started</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.this_documentation">This
      Documentation</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.compilers_and_platforms">Compilers
      and Platforms</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/getting_started.html#boost_localfunction.getting_started.installation">Installation</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_localfunction/tutorial.html">Tutorial</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.local_functions">Local Functions</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.Binding">Binding Variables</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.binding_the_object__this_">Binding
      the Object <code class="computeroutput"><span class="keyword">this</span></code></a></span></dt>
<dt><span class="section"><a href="boost_localfunction/tutorial.html#boost_localfunction.tutorial.templates">Templates</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html">Advanced Topics</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.default_parameters">Default
      Parameters</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros">Commas
      and Symbols in Macros</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns">Assignments
      and Returns</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.nesting">Nesting</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_">Accessing
      Types (concepts, etc)</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_">Specifying
      Types (no Boost.Typeof)</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.inlining">Inlining</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.recursion">Recursion</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.overloading">Overloading</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications">Exception
      Specifications</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers">Storage
      Classifiers</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions">Same
      Line Expansions</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_">Limitations
      (operators, etc)</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_localfunction/examples.html">Examples</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_">GCC
      Lambdas (without C++11)</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.constant_blocks">Constant
      Blocks</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.scope_exits">Scope Exits</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.boost_phoenix_functions">Boost.Phoenix
      Functions</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.closures">Closures</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.gcc_nested_functions">GCC
      Nested Functions</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/examples.html#boost_localfunction.examples.n_papers">N-Papers</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_localfunction/alternatives.html">Annex: Alternatives</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/no_variadic_macros.html">Annex: No Variadic
    Macros</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/implementation.html">Annex: Implementation</a></span></dt>
<dt><span class="section"><a href="reference.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="reference.html#header.boost.local_function_hpp">Header &lt;boost/local_function.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="reference.html#header.boost.local_function.config_hpp">Header &lt;boost/local_function/config.hpp&gt;</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_localfunction/release_notes.html">Release Notes</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/bibliography.html">Bibliography</a></span></dt>
<dt><span class="section"><a href="boost_localfunction/acknowledgments.html">Acknowledgments</a></span></dt>
</dl>
</div>
<p>
    This library allows to program functions locally, within other functions, and
    directly within the scope where they are needed.
  </p>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_localfunction.introduction"></a><a class="link" href="index.html#boost_localfunction.introduction" title="Introduction">Introduction</a>
</h2></div></div></div>
<p>
      <span class="emphasis"><em>Local functions</em></span> (a.k.a., <a href="http://en.wikipedia.org/wiki/Nested_function" target="_top"><span class="emphasis"><em>nested
      functions</em></span></a>) are a form of <span class="emphasis"><em>information hiding</em></span>
      and they are useful for dividing procedural tasks into subtasks which are only
      meaningful locally, avoiding cluttering other parts of the program with functions,
      variables, etc unrelated to those parts. Therefore, local functions complement
      other structuring possibilities such as namespaces and classes. Local functions
      are a feature of many programming languages, notably <a href="http://en.wikipedia.org/wiki/Nested_function#An_example" target="_top">Pascal</a>
      and <a href="http://en.wikipedia.org/wiki/Nesting_(computing)#In_programming" target="_top">Ada</a>,
      yet lacking from <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
      (see also <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2511.html" target="_top">[N2511]</a>).
    </p>
<p>
      Using <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
      lambda functions</a>, it is possible to implement local functions by naming
      lambda functions assigning them to local variables. For example (see also
      <a href="../../example/add_cxx11_lambda.cpp" target="_top"><code class="literal">add_cxx11_lambda.cpp</code></a>):
    </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span>                            <span class="comment">// Some local scope.</span>
    <span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>               <span class="comment">// Variables in scope to bind.</span>

    <span class="keyword">auto</span> <span class="identifier">add</span> <span class="special">=</span> <span class="special">[</span><span class="identifier">factor</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">sum</span><span class="special">](</span><span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>    <span class="comment">// C++11 only.</span>
        <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
    <span class="special">};</span>

    <span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>                                 <span class="comment">// Call the lambda.</span>
    <span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">};</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span>     <span class="comment">// Pass it to an algorithm.</span>

    <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">60</span><span class="special">);</span>                  <span class="comment">// Assert final summation value.</span>
    <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
    </p>
<p>
      This library allows to program local functions portably between <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
      and <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> (and
      with performances comparable to lambda functions on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
      compilers). For example (see also <a href="../../test/add.cpp" target="_top"><code class="literal">add.cpp</code></a>):
    </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span>                            <span class="comment">// Some local scope.</span>
    <span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>               <span class="comment">// Variables in scope to bind.</span>

    <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
        <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
    <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>

    <span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>                                 <span class="comment">// Call the local function.</span>
    <span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">};</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span>     <span class="comment">// Pass it to an algorithm.</span>

    <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">60</span><span class="special">);</span>                  <span class="comment">// Assert final summation value.</span>
    <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
    </p>
<p>
      This library supports the following features for local functions:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Local functions can capture, or better <a href="http://en.wikipedia.org/wiki/Name_binding" target="_top"><span class="emphasis"><em>bind</em></span></a>,
          any of the variables from the enclosing scope (a function together with
          its captured variables is also called a <a href="http://en.wikipedia.org/wiki/Closure_(computer_science)" target="_top"><span class="emphasis"><em>closure</em></span></a>).
        </li>
<li class="listitem">
          The local function body is programmed using the usual C++ statement syntax
          (as a consequence, compiler errors and debugging retain their usual meaning
          and format).
        </li>
<li class="listitem">
          Local functions can be passed as template parameters so they can be conveniently
          used with STL algorithms and other templates. <a href="#ftn.boost_localfunction.introduction.f0" class="footnote" name="boost_localfunction.introduction.f0"><sup class="footnote">[1]</sup></a>
        </li>
<li class="listitem">
          However, local functions must be specified within a declarative context
          (e.g., at a point in the code where local variables can be declared) thus
          they cannot be specified within expressions. <a href="#ftn.boost_localfunction.introduction.f1" class="footnote" name="boost_localfunction.introduction.f1"><sup class="footnote">[2]</sup></a>
        </li>
</ul></div>
<p>
      See the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a>
      section for a comparison between this library, <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
      lambda functions</a>, <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>,
      and other C++ techniques that implement features related to local functions.
    </p>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.boost_localfunction.introduction.f0" class="footnote"><p><a href="#boost_localfunction.introduction.f0" class="para"><sup class="para">[1] </sup></a>
            This is a strength with respect to <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
            functors implemented using local classes which cannot be passed as template
            parameters (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a>
            and the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a>
            section).
          </p></div>
<div id="ftn.boost_localfunction.introduction.f1" class="footnote"><p><a href="#boost_localfunction.introduction.f1" class="para"><sup class="para">[2] </sup></a>
            This is a weakness with respect to <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
            lambda functions</a> which can instead be specified also within expressions
            (see the <a class="link" href="boost_localfunction/alternatives.html" title="Annex: Alternatives">Alternatives</a>
            section).
          </p></div>
</div>
</div>
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